The present invention relates to a barrel-type multistage pump used in relatively high-lift applications.
A general structure of diffusers and stages of a conventional barrel-type multistage pump is shown in FIG. 1. In the centrifugal multistage pump, the kinetic energy of a fluid flowing out of impellers 1 in the centrifugal direction is converted into a pressure energy at an enlarged flow channel of diffusers 16 with blades provided at outer circumferences of the impellers, the direction of the fluid is turned to the inside in the radial direction at a U-turn passage 17 formed at each stage on the outer circumferential side of the diffusers, and then the fluid is guided to the impeller in the next stage by using return vanes 20 provided on the downstream side of the U-turn passage 17. In the last stage, the fluid discharged from the entire circumferences of the diffusers 16 is fed to a discharge pipe 41 via a connecting channel 19 and a rotating flow channel 18. The meridional plane of the connecting channel 19 is provided in a direction orthogonal to a rotary shaft 10, namely, the meridional plane of the connecting channel 19 is linearly provided in the outer circumferential direction, a junction part between the connecting channel 19 and the rotating flow channel 18 is provided at a position apart from a center line 41a of the discharge pipe 41. The object of providing the junction part at a position apart from the center line 41a of the discharge pipe 41 is to shorten the length of the pump in the axial direction by shifting the position of the discharge pipe 41 to the side of a suction opening 3, and to reduce the cost by reducing the size and weight of the pump. However, a fluid loss occurs at a position where the fluid is discharged in the above-described configuration, and there are two possible factors of the fluid loss.
As a first factor, the fluid in the connecting channel 19 flows into the rotating flow channel 18, the most of the fluid flows out after being swirled at an area X near a junction part, and then the fluid flows out to the discharge pipe 41 in the shape of the last stage as shown in FIG. 3. When viewed from the cross-sectional direction of the rotating flow channel 18, the velocity of the rotating flow is high near the junction part X with the connecting channel 19, whereas the velocity thereof is low near a position Y on the suction side of the pump that is apart from the junction part X. Accordingly, the velocity non-uniformity on the cross-section causes a fluid loss. Further, as the flow near this position on the cross-section of the meridional plane, the direction of the fluid flowing out of the connecting channel 19 is once turned to the axial direction of the pump at the rotating flow channel 18 as shown in FIG. 4, and is further turned to the outer circumferential direction again at a position reaching the discharge pipe 41. As a result, the fluid reaches the discharge pipe 41 so as to pass through a crank. In particular, when the fluid flows into the discharge pipe 41, the direction of the flow that is originally fast in the circumferential direction is further turned to a direction orthogonal to the axial direction. Thus, the fluid cannot flow along the shape at this position, and is largely separated and disordered, leading to an enormous fluid loss.
As a second factor, as shown in FIG. 5 that is viewed from the cross-sectional direction passing through the connecting channel 19 and the discharge opening 4 shown in FIG. 4, when the fluid flowing out of the impellers 1 passes through a guide impeller blade 16a provided at the diffuser 16 to reach the rotating flow channel 18 via the connecting channel 19, all the flow does not reach the discharge pipe 41 after flowing out in one direction of the rotating flow channel 18 in the rotational direction of the impellers. Near the discharge pipe 41, in particular, it has been found that a part of the fluid flowing out of the connecting channel 19 flows in a direction opposed to the rotational direction (indicated by the arrows) of the impellers 1 under the influence of the disorder of the flow near the discharge pipe 41, and interferes with the fluid flowing in the forward direction of the rotating flow channel before reaching the discharge pipe. An enormous flow loss occurs also at the position.
The second factor is possibly and mainly derived from the fact that the cross-sectional area of the rotating flow channel 18 is constant in the circumferential direction, and the amount of flow flowing into the rotating flow channel 18 from the connecting channel 19 is constant in the circumferential direction. Thus, the velocity of flow in the rotating flow channel 18 in the rotational direction of the impellers is increased in the rotational direction of the impellers 1 from a connecting part between the rotating flow channel 18 and the discharge pipe 41, and then the fluid flows out via the discharge pipe 41. However, near the downstream side of the impellers 1 in the rotational direction from the connecting part between the rotating flow channel and the discharge pipe 41 in the rotating flow channel 18, the velocity of the flow in the circumferential direction is significantly lowered, and the fluid cannot smoothly flow. Thus, the fluid flows in a direction opposed to the swirl direction at this position.
Japanese Patent Application Laid-Open No. H11-303796 proposes a vortex pump or a radial flow pump in which the cross-sectional area of a rotating flow channel is gradually increased in the rotational direction of impellers from a position apart from a discharge opening to the discharge opening, so that the cross-sectional area of the rotating flow channel is gradually increased in the same direction. In addition, Japanese Patent Application Laid-Open No. 2006-152849 proposes a centrifugal pump in which a spiral-shaped groove that is gradually deepened towards a discharge opening is provided, in the rotational direction, from a circular part between an outer circumferential edge of an inner wall surface of a discharge casing and a circular arc corresponding to an outer circumferential circle of impellers. These are provided to contribute to reduction in energy consumption while the flow in the rotating flow channel is rectified and a loss in the flow channels, inside the pump is reduced to improve the efficiency of the pump.